Display units often incorporate pointer displays having a rotatable illuminated pointer arm which extends radially outwards from an axially extending rotatable shaft. The shaft is driven by a motor, which is normally concealed from view beneath a display area above which the pointer arm moves. Light is introduced into the base of the pointer arm and is transmitted along the length of the arm, which therefore acts as a light pipe. One common way in which the illumination is directed forwards to a viewer of the display unit is by a painted rear surface on the pointer arm. Along the length of the arm the painted rear surface scatters some light to the viewer.
One way of introducing light into the pointer arm is to use the pointer shaft also as a light pipe. A shaft, in this disclosure, may be replaced by a stem. The light pipe shaft can extend through the motor so that a source of illumination can introduce light into a rearmost end face of the light pipe shaft. The light pipe shaft then includes a mirror or light splitter to redirect at least some of the light from the shaft into the pointer arm.
Disclosed herein is a pointer display having two such illuminated pointer arms which rotate about a common axis. Such coaxial pointer displays are referred to herein as dual rotatable pointer displays.
An example of such a display unit is an automotive instrument is a clock which has two hands driven by concentric drive shafts. In many clocks, these shafts are geared together to obtain the correct ratio of elapsed hours and minutes. It can, however, be desirable if the pointer arms, here the two hands of the clock, are moved independently, each by its own control motor. Another example in the automotive field is a combined fuel gauge and temperature gauge with two independent pointers that share a common axis.
In automotive applications, it may not be economic or practical to use dual output motors having two concentric shafts, and such dual output motors may make it difficult or impossible to illuminate both pointer arms using a common light source.
One way of arranging such a dual rotatable pointer display such that both pointer arms are illuminated with a common light source is disclosed in patent document US 2002/0108555 A1. In this arrangement, there are two side-by-side motors behind the display area which separately drive two rotatable shafts, a first one of which of which is on the rotation axis and a second one of which is tubular and concentric with the rotation axis. The first shaft drives a first pointer arm and the second shaft drives a second pointer arm. Light is introduced into a rear end face of the axial first shaft, which includes a light splitter to redirect proportions of the light into both pointer arms.
Although this dual rotatable pointer display provides independent rotation for two illuminated pointer arms sharing a common light source, this arrangement is mechanically complex, requiring two separate motor units each of which includes a worm drive mechanism that drives a gear on a portion of the corresponding shaft.